This application claims the priority of German patent document 102 33 821.3, filed 25 Jul. 2002 (PCT International Application No. PCT/EP2003/007030), the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for controlling a hybrid power supply system in a mobile device having at least one electric drive motor, a fuel cell system and a dynamic power system. The electrical outputs of the fuel cell system are connected to one side of a power converter whose other side feeds the drive motor which is controlled by a motor control unit. The dynamic power system has a storage battery which is connected to one side of a d.c./d.c. converter whose other side is connected to the electrical outputs of the fuel cell system and to one side of the power converter.
Power supply systems for an electric vehicle which has a fuel cell and a storage battery that can be connected in parallel with it are known. The electrical outputs of the fuel cell are connected to a motor for driving the vehicle, and to a d.c./d.c. converter to which auxiliary machines in the vehicle are connected. The power supply system contains a residual charge monitoring device for measuring the residual charge of the storage battery when the power supply system stops operation. If the residual charge is then less than a predefinable limiting value, the fuel cell charges the storage battery to the limiting value. The power supply system is not shut down until after such charging is completed. (See German patent document DE 197 31 250 A1.)
Hybrid drive vehicles which have, as a power source, a fuel cell that feeds an electric drive motor are known. Such vehicles contain a power accumulator and electrical secondary loads. Two separate circuits which are provided, with switching devices for optionally connecting the motor and the secondary load to the fuel cell or to the power accumulator, and a switchable connection between the fuel cell and the power accumulator are provided in the vehicle (German patent document DE 198 10 467 C1).
European patent document EP 0 334 474 B1 discloses a fuel cell system which includes a fuel tank, a reformer, a fuel cell, and a d.c./d.c. converter that is connected to the electrical outputs of the fuel cell. A load is connected to the d.c./d.c. converter and a storage battery is connected in parallel therewith. The fuel cell system contains a control unit which senses residual charge of the storage battery, and ensures that the storage battery is charged to a predefinable charge state in the shortest possible time.
Finally, U.S. Pat. No. 5,714,874 discloses a power supply system that comprises a fuel cell, a d.c./d.c. converter which is connected to the electrical outputs of the fuel cell, and a load which is connected to the outputs of the d.c./d.c. converter. A storage battery is connected in parallel with the load. A control unit regulates the current flowing via the d.c./d.c. converter such that the output voltage of the fuel cell remains within a predefined range.
One object of the invention is to provide a method and apparatus for controlling a power supply of a mobile device having at least one electric drive motor and a hybrid power system composed of a fuel cell system and a dynamic power system, such that the hybrid power system operates in an optimum way in terms of the required dynamics, with adaptation to the parameters and/or operating states.
This and other objects and advantages are achieved by the method according to the invention in which, for a particular setpoint power request, multiple signals are processed to determine the components of the requested setpoint power that will be provided, respectively, by the fuel cell system and by the dynamic power system. The signals which are processed for this purpose include signals generated by a signal transmitter for requesting the setpoint power of the drive motor, a signal from an operating mode switch (which has a plurality of selectable settings that are assigned to different types of dynamic behavior of the device), signals from power sensors for the output current and voltage of the fuel cell and signals from a sensor for the velocity of the mobile device. When there is a change in the setpoint power, the difference between the partial power that can be generated by the fuel cell system (with a delay according to the transition function) and the setpoint power is provided by the storage battery of the dynamic power system, by applying corresponding setpoint values to the d.c./d.c. converter, considering the power of the drive motor which has already been output and the power of the fuel cell system which has already been generated as well as the velocity of the device, and taking into account the selected type of dynamic behavior and the different transition functions of the fuel cell system and of the dynamic power system. Based on the power demand of the mobile device (which depends on the setpoint value of the torque, the setting element and the measured values of the sensors, and which is calculated from the signals, position sensors and measured values), the power demand can be adapted to the fuel cell system in a suitable way, (i.e., with a favorable efficiency and/or favorable time behavior), while the storage battery supplies the power contribution for rapid dynamics of the mobile device.
In the case of a sudden increase or decrease in the setpoint power, the resultant corresponding increase or decrease in the current flowing out of or into the storage battery via the d.c./d.c. converter is limited in particular to a maximum prescribable discharge current or charge, determined, for example, by the type of the storage battery used.
In one preferred embodiment, from a vehicle control unit of the mobile device load, current values of additional loads in the device are superimposed on the power demand values for the drive motor which are determined from the setpoint power. The result is fed (with a charge current value generated when necessary by a battery management system) to a power control unit with limitation to a predeterminable fuel cell maximum power value of a power control unit. The power control unit also receives velocity values, torque setpoint values from a setting element, battery charge state values and the setting of the operating mode selector switch. As a function of the fed values, the power control unit calculates the values of the overall power demand, portions which is to be contributed by the fuel cell system taking into account its inertia behavior and the selected dynamic behavior, and outputs corresponding setpoint values to the actuating elements of the fuel cell system. In each case the values of the current which is output by the fuel cell are determined and subtracted from the value of the current required by the drive motor; and the result is fed as current setpoint values to the d.c./d.c. converter with limitation to a maximum specifiable discharge current or charge current of the storage battery. The power control unit detects, by reference to the values fed to it, the operating mode and the operating state of the mobile device and concludes therefrom the type of current contribution which the fuel cell system must make for the power converter and the auxiliary drive. The storage battery provides the current contributions for the high-speed dynamic demand levels. The method according to the invention permits rapid setting of the current in order to achieve satisfactory driving dynamics, by the d.c./d.c. converter.
In a further favorable embodiment, the sum of the current which is drawn from the drive motor (via the power converter) and the currents which are drawn from the other loads of the device are subtracted from the value of the current that is output by the fuel cell, and when a maximum predefinable value of the discharge current of the storage battery is reached, it is limited to its discharge current. The result of the difference between the values of the currents which are drawn from the further loads is added to the value of the available fuel cell current and signaled to the control unit of the device as an available value of the current. The available current is a dynamic current in response to the demand for a current. The fuel cell system meters the amount of fuel sufficient for this current to be drawn. The control unit is therefore capable of matching the current demands made of the mobile device to the respective available values of the current.
Its particularly advantageous, that the response function of the fuel cell system is simulated as a controlled system, using a memory element of the n-th order. The torque setpoint value which is output by the vehicle control unit of the mobile device is applied to the memory element and to a control unit for the dynamic power system, and the values generated according to the response function of the controlled system are fed to the control unit. The current which is to be applied by the dynamic power system as a current setpoint value is supplied to the d.c./d.c. converter by the control unit by means of a limiter element with a ramp, the gradient of which can be set to at least two values as a function of control signals from the device. This embodiment achieves especially good overall dynamics, and the efficiency of the dynamic power system is exploited to an optimum degree. For example, during rapid starting of the mobile device (i.e., at the commencement of starting and with low power of the fuel cell system), the power is applied by the dynamic power system so that the large torque necessary to accelerate the device is rapidly available. At high rotational speeds, the power for the acceleration in order to reach a high rotational speed is output by the dynamic power system. A sliding transition of the power contributions of the fuel cell and power system is achieved by means of the power control unit.
In order to achieve high acceleration during an acceleration of the device (when the setpoint torque is determined by the vehicle control unit by pilot control and a maximum current for the generation of the setpoint torque is determined from a characteristic diagram with the torque as a function of the maximum current and the rotational speed), the difference between the current generated by the fuel cell system during the acceleration and the overall current which is required by the dynamic power system according to the characteristic diagram in order to achieve the high acceleration, is generated. With this embodiment, particularly good longitudinal dynamics are generated in a mobile device, in particular an electric vehicle, since utilization of the dynamic power system is optimized.
In order to utilize the power of the mobile device satisfactorily, the excess energy occurring when the load of the drive motor is reduced is recovered and stored in the dynamic power system.
The d.c./d.c. converter is also set in such a way that it feeds current into the storage battery and charges it when there is a negative load jump, (i.e., due to a corresponding change in the actuating element for the power to be output by the drive motor, the power converter is set to reverse mode). The charge current is determined by the charge controller which controls the charge currents across the d.c./d.c. converter as a function of the charge state of the storage battery. When there is a reduction in the setpoint torque to be output by the drive motor as a result of the presetting of a lower torque setpoint value, the current which is necessary for the lower torque is preferably determined from the characteristic diagram. With reference to the current load state of the fuel cell system, given the presetting of the lower torque setpoint value and the storage capacity of the storage battery, the latter is charged with the maximum permissible charge current by means of the d.c./d.c. current after the reversal of the flow of current in the power converter, and the fuel cell system is set to the current which is necessary for the lower setpoint torque. This measure avoids the risk of overheating of the fuel cell system.
In another preferred embodiment, the direction of the supply of combustion gas and air to the fuel cell is reversed periodically, and during such reversal, a current pulse matched to the instantaneous output of current of the fuel cell system and/or of the dynamic power system directly before the changeover is fed in to the power converter by the dynamic power system via the d.c./d.c. transformer. This avoids undesired fluctuations in the drive torque.
It is also expedient to monitor the output voltage of the fuel cell system to determine when a voltage limiting value that is permissible for satisfactory operation is reached or undershot. When the voltage limiting value is reached, the voltage in the power system which is connected to the output of the fuel cell is regulated to at least the permissible limiting value by feeding in current via the d.c./d.c. current. In particular, the load situation of the power supply system during the intervention of the regulating process and the frequency of intervention of the voltage regulating process during the operation of the power supply system are registered. After a predefinable number of interventions have been exceeded the dynamics are limited by reducing the rate of increase in the current of the fuel cell system and/or the dynamic power system and the magnitude of the power which is output.
It is advantageous to limit the rate of increase in the output power of the fuel cell system given sufficient storage battery charge when the torque setpoint value is increased, and to supply the current necessary to output the torque setpoint value, from the dynamic power system during the increase in the output power. In this context it is advantageous for operation of the power supply system with a high level of efficiency to approach the load state of the fuel cell system which is demanded by the torque setpoint value by means of a ramp with a low rate of increase.
It is particularly expedient if at least three operating modes for the drive motor can be set by means of the operating mode selector switch, one operating mode of which is aimed at a high level of dynamics of the mobile device, a second of which is aimed at a low level of dynamics with high efficiency and a third of which is aimed at a stop and go operating mode. When accelerations occur in the stop and go operating mode, currents are generated for the drive motor by the dynamic power system and stored therein during braking.
The portion of the current to be supplied by the dynamic power system which is formed by the current necessary to generate a requested drive power, in particular with the existing actual value of the current consumed by the mobile device, and the current available from the fuel cell system is determined.
In the case of reduced power of the fuel cell system, an emergency operating mode of the power supply system is preferably ensured by a voltage regulating mode in the high voltage power system, by means of the d.c./d.c. converter and by supplying current from the storage battery.
In the power supply control apparatus according to the invention, a vehicle control unit is connected to a velocity sensor of the mobile device and to a signal transmitter for a setpoint torque (to be generated by the drive motor). The vehicle control unit establishes the setpoint torque for a motor control unit, and determines the current setpoint values for the mobile device which are stored in a characteristic diagram for torque values and rotational speed values. The vehicle control unit is connected to the power control unit which is connected to the fuel cell system, a battery management system for the storage battery and to the d.c./d.c. converter. The current output by the fuel cell is measured and is fed as a fuel cell current value to the power control unit. The current of the drive motor is measured upstream of the power converter and is fed as a driving current value to the power control unit. The currents of the other loads are measured or calculated and fed to the power control unit as a composite current value. An operating mode selector switch for setting various types of dynamic behavior of the power supply system is connected to the power control unit. Values relating to the charge state of the storage battery, from the battery management system, and values relating to the maximum prescribable charge current and discharge currents are fed to a power flux controller in the power control unit. The power setpoint value, the fuel cell current value, the driving current value, the composite current value, the operating mode set with the operating mode selector switch, the charge state value and the maximum prescribed values of the charge and discharge currents are processed in the power control unit and in the associated power flux controller, with one or more programs. When there is a change in the setpoint power, the difference between the partial power that can be generated by the fuel cell system (with a delay according to the response function) and the setpoint power, is supplied by the storage battery of the dynamic power system by applying corresponding setpoint values to the d.c./d.c. converter, considering the power of the drive motor which has already been output and the power of the fuel cell system which has already been generated as well as the velocity of the device, and taking into account the selected type of dynamic behavior and the different response functions of the fuel cell system and of the dynamic power system. The current which is to be contributed by the battery is determined from the vehicle current and the available current of the fuel cell, which takes only a very short time.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.